linux-kernel - Re: [PATCH v5 7/7] iommu/exynos: Use device dependency links to control runtime pm

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Date:   Thu, 10 Nov 2016 00:55:26 +0100
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Lukas Wunner <lukas@...ner.de>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-samsung-soc@...r.kernel.org, Joerg Roedel <joro@...tes.org>,
        Inki Dae <inki.dae@...sung.com>, Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Kevin Hilman <khilman@...nel.org>,
        Tobias Jakobi <tjakobi@...h.uni-bielefeld.de>,
        Tomasz Figa <tomasz.figa@...il.com>,
        Grant Likely <grant.likely@...retlab.ca>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [PATCH v5 7/7] iommu/exynos: Use device dependency links to
 control runtime pm

On Tue, Nov 08, 2016 at 04:30:44PM +0100, Lukas Wunner wrote:
> On Tue, Nov 08, 2016 at 08:27:12AM +0100, Marek Szyprowski wrote:
> > On 2016-11-07 22:47, Luis R. Rodriguez wrote:
> > > Has there been any review of the existing similar solutions out there
> > > such as the DRM / audio component framework? Would that help ?
> > 
> > Nope, none of that solution deals with runtime pm.
> 
> Well, they do.  Hybrid graphics laptops often have an HDA controller
> on the discrete GPU and I assume that's what Luis meant. There's code
> in drivers/gpu/vga/vga_switcheroo.c to make this (only sort of) work:
> 
> * When the GPU is powered up/down, the HDA controller's driver is
>   instructed to pm_runtime_get/put the HDA device (see call to
>   set_audio_state() in vga_switcheroo_set_dynamic_switch()).
> 
> * When a runtime PM ref is acquired on the HDA device, the
>   GPU is powered up (see vga_switcheroo_runtime_resume_hdmi_audio()).
> 
> 
> Unfortunately this is all fairly broken, e.g.:
> 
> * If a runtime PM ref on the HDA device is held for more than 5 sec
>   (autosuspend delay of the GPU), the GPU will be powered down and
>   the HDA device will become inaccessible, regardless of the runtime
>   PM ref still being held (because vga_switcheroo_set_dynamic_switch()
>   doesn't check the refcount of the HDA device).
> 
> * The DRM device is afforded direct-complete but the HDA device is not.
>   If the GPU is handled earlier by dpm_suspend(), then runtime PM will
>   have been disabled on the GPU and thus the HDA device will fail to
>   runtime resume before system sleep.
> 
> Rafael's series allows representation of such inter-device dependencies
> in the PM core and can thus replace kludgy and broken "solutions" like
> the one above.
> 
> There's one thing that I haven't understood myself though:  In an e-mail
> exchange in September Rafael has argued that the above-mentioned hybrid
> graphics use case "isn't a good [example] IMO.  That clearly is a case
> when two (or more) devices share power resources controlled by a single
> on/off switch.  Which is a clear use case for a PM domain."
> 
> The same seems to apply to Marek's SYSMMU use case.  When applying device
> links to SYSMMU or hybrid graphics, we select one of the devices in the
> PM domain as master and have the other one depend on it as slave, i.e.
> a synthetic hierarchical relationship is established.
> 
> I've responded to Rafael on September 18 that this can't be solved with
> a struct dev_pm_domain, but haven't received a reply since:
> https://lkml.org/lkml/2016/9/18/103

Rafael note:

The one he asked here.

  Luis